Back to basics: DIDs, DDIs and Inbound Numbers

After you’ve used VOIP for a little while it’s easy to forget how complicated things can be when just starting out. I thought I’d write a series of brief posts explaining some of the basics.

This post is about Inbound Numbers, and how incoming calls get to your VOIP equipment/server.

What numbers are available?

You can get telephone numbers all over the world. It’s easier to get numbers in some countries than others. You can also get different types of numbers. For example; in the UK you can get geographic numbers, freephone numbers, national numbers and even mobile numbers.

DID or DDI, which is it?

In the UK we tend to call Inbound numbers DDIs (direct dial-in), the US and other countries call them DIDs (direct inward dialing)

Where do I get an Inbound Number?

There are literally hundreds of call providers that can supply Inbound Numbers. You should choose your provider carefully as it’s more critical to find a reliable number provider, than a reliable outbound call provider. It’s very easy to switch to a new outbound call provider but once you have published your incoming phone number it’s more difficult to move that to a new call provider. Sometimes impossible.

Call providers are referred to as ITSPs (Internet Telephony Service Provider).

How do the calls get to my Voice Server/PBX?

Calls initially go to you Inbound Number provider. They are then delivered to your PBX/Voice Server. This happens very quickly. In the world of Asterisk calls are delivered to your server using either SIP or IAX2. Both of these are VOIP standards. The most common standard is SIP and you will find that most ITSPs can deliver the calls to your Asterisk server using SIP.

How much do they cost?

The cost of numbers vary wildly from country to country. Geographic numbers in the UK are very inexpensive, you can ever get free ones. Numbers in other countries can be more expensive. Normally when you buy a number you will pay a fixed monthly fee. This usually includes a number of concurrent channels, plus a number of included minutes. Once you exceed your monthly minute allocation there is sometimes a per minute cost for additional minutes. If you expect more than a couple of concurrent calls and/or lots of calls to your number then check these 2 restrictions carefully when looking for a number.

For freephone numbers you are likely to get no included minutes and pay a per minute rate for all incoming calls.

Can I get a number in any country?

Not necessarily. For instance, anyone can buy a number in the UK or US. However, to buy a number in France you need to prove that you are a resident by providing a utility bill or similar.

What happens to an inbound call when it gets to my Voice Server?

In FreePBX you control what happens to an incoming call by creating an Inbound Route. It could go to an extensions, voicemail, interactive menu, conference room or one of many other options.